1. Field of the Invention
The present invention relates to monolithic semiconducting ceramic electronic components, and in particular, the invention relates to a semiconducting ceramic component having barium titanate as a major constituent and having a positive temperature coefficient of resistance.
2. Description of the Related Art
Conventionally, barium titanate-based semiconducting ceramics have been widely used for applications such as temperature control, overcurrent protection, and isothermal heating because barium titanate-based semiconducting ceramics have positive resistance temperature characteristics (hereinafter referred to as xe2x80x9cPTC characteristicsxe2x80x9d) in which the resistivity is low at room temperature and the resistance abruptly increases at a temperature higher than the Curie Point. In particular, low room temperature resistance is desired in electronic components for overcurrent protection of circuits. In Universal Serial Bus (USB) computer peripheral equipment, small semiconducting ceramic components having low resistivity and high withstand voltage are required.
In response to such demands, a monolithic semiconducting ceramic electronic component is disclosed in Japanese Unexamined Patent Publication No. 57-60802. In the monolithic semiconducting ceramic electronic component, semiconducting ceramic layers having barium titanate as a major constituent and internal electrode layers composed of a Ptxe2x80x94Pd alloy are alternately deposited and integrally fired. By constructing such a multi-layered structure, the electrode area in the semiconducting ceramic electronic component greatly increases, and the size of the electronic component itself can be reduced. However, it is difficult to obtain ohmic contact between the internal electrode layers and the semiconductor layers in the monolithic semiconducting ceramic electronic component, resulting in a large increase in resistance at room temperature.
A monolithic semiconducting ceramic electronic component is also disclosed in Japanese Unexamined Patent Publication No. 6-151103 in which a Ni-based metal is used as a material for internal electrodes instead of the Ptxe2x80x94Pd alloy. The material for internal electrodes using the Ni-based metal is oxidized if fired in air, and therefore, after being fired in a reducing atmosphere, the material must be subjected to reoxidation treatment at a temperature which does not oxidize the Ni-based metal. Since ohmic contact between the internal electrodes and semiconducting ceramic layers can be obtained, an increase in resistance at room temperature can be avoided. However, since the reoxidation treatment at low temperatures is required to prevent the Ni-based metal from oxidizing, the width of resistivity variation is small at less than 2 units.
A monolithic semiconducting ceramic electronic component is also disclosed in Japanese Unexamined Patent Publication No. 1-11302 in which the average particle size of a semiconducting ceramic and the thickness of a semiconducting ceramic layer are taken into consideration. In the monolithic semiconducting ceramic electronic component, the thickness of the semiconductor layer is at least 5 times the average particle size of the semiconducting ceramic, and the average particle size of the semiconducting ceramic is 1 to 30 xcexcm. By constructing such a structure, semiconducting ceramic layers and internal electrodes can be brought into ohmic contact with each other and degradation of the PTC characteristics can be avoided. However, the ceramic electronic component has an insufficient withstand voltage, resulting in problems in practical use.
It is an object of the present invention to provide a monolithic semiconducting ceramic electronic component in which the size of the electronic component itself can be reduced, the room temperature resistance is as low as about 0.2 xcexa9 or less, the width of resistivity variation is about 2.5 units or more, and the withstand voltage is as high as about 10 V or more.
The present invention has been achieved in view of the object described above.
In a first aspect of the present invention, a monolithic semiconducting ceramic electronic component includes barium titanate-based semiconducting ceramic layers and internal electrode layers, which are alternately deposited, and external electrodes electrically connected to the internal electrode layers. The semiconducting ceramic layers contain ceramic particles having an average particle size of about 1 xcexcm or less, and an average number of ceramic particles per layer in the direction perpendicular to the semiconducting ceramic layers is about 10 or more.
By constructing such a structure, the size will be reduced, and the semiconducting ceramic electronic component will have low resistance at room temperature, large width of resistivity variation and a high withstand voltage. That is, by setting the average particle size at about 1 xcexcm or less, the withstand voltage can be improved. Since a larger number of ceramic particles are present per layer, the semiconducting ceramic layers can be thinner. By setting the average number of ceramic particles per layer in the direction perpendicular to the semiconducting ceramic layers at about 10 or more, an increase in the resistance at room temperature due to diffusion of internal electrode constituents into the semiconducting ceramic layers can be avoided.
In a second aspect of the present invention, the internal electrode layers are preferably composed of a nickel-based metal in the monolithic semiconducting ceramic electronic component.
By using the nickel-based metal as a material for the internal electrode layers, the semiconducting ceramic layers and the internal electrode layers are securely brought into ohmic contact with each other, thus enabling one to avoid an increase in resistance at room temperature and to increase the width of resistivity variation in the semiconducting ceramic electronic component. Even if reoxidation treatment is performed at low temperatures in order not to oxidize the internal electrodes composed of the nickel-based metal, the width of resistivity variation in the semiconducting ceramic electronic component can be increased.